diff --git a/dvc/scm/git/__init__.py b/dvc/scm/git/__init__.py
index 17c1067e9..186fad525 100644
--- a/dvc/scm/git/__init__.py
+++ b/dvc/scm/git/__init__.py
@@ -254,8 +254,8 @@ class Git(Base):
                 commits.append(str(a_commit))
                 commits.append(str(b_commit))
             else:
-                a_commit = self.git.head.commit
-                b_commit = self.git.commit(a_ref)
+                a_commit = self.git.commit(a_ref)
+                b_commit = self.git.head.commit
                 commits.append(str(a_commit))
                 commits.append(str(b_commit))
             trees[DIFF_A_TREE] = self.get_tree(commits[0])
diff --git a/tests/test_diff.py b/tests/test_diff.py
index b9c6d5975..37e2af358 100644
--- a/tests/test_diff.py
+++ b/tests/test_diff.py
@@ -27,22 +27,22 @@ class TestDiff(TestDvc):
     def test(self):
         out = self.dvc.scm.get_diff_trees(self.a_ref)
         self.assertFalse(out[git.DIFF_EQUAL])
-        self.assertEqual(str(self.a_ref), out[git.DIFF_B_REF])
-        self.assertEqual(str(self.git.head.commit), out[git.DIFF_A_REF])
+        self.assertEqual(str(self.a_ref), out[git.DIFF_A_REF])
+        self.assertEqual(str(self.git.head.commit), out[git.DIFF_B_REF])
 
 
 class TestDiffRepo(TestDiff):
     def test(self):
         result = self.dvc.diff(self.new_file, a_ref=self.a_ref)
         test_dct = {
-            git.DIFF_B_REF: str(self.a_ref),
-            git.DIFF_A_REF: str(self.git.head.commit),
+            git.DIFF_A_REF: str(self.a_ref),
+            git.DIFF_B_REF: str(self.git.head.commit),
             git.DIFF_LIST: [
                 {
                     git.DIFF_TARGET: self.new_file,
-                    git.DIFF_OLD_FILE: self.new_file,
-                    git.DIFF_OLD_CHECKSUM: "c81fec3d710806070bcd67f182d1279c",
-                    git.DIFF_SIZE: -13,
+                    git.DIFF_NEW_FILE: self.new_file,
+                    git.DIFF_NEW_CHECKSUM: "c81fec3d710806070bcd67f182d1279c",
+                    git.DIFF_SIZE: 13,
                 }
             ],
         }
@@ -76,34 +76,34 @@ class TestDiffDir(TestDvc):
     def test(self):
         out = self.dvc.scm.get_diff_trees(self.a_ref)
         self.assertFalse(out[git.DIFF_EQUAL])
-        self.assertEqual(str(self.a_ref), out[git.DIFF_B_REF])
-        self.assertEqual(str(self.git.head.commit), out[git.DIFF_A_REF])
+        self.assertEqual(str(self.a_ref), out[git.DIFF_A_REF])
+        self.assertEqual(str(self.git.head.commit), out[git.DIFF_B_REF])
 
 
 class TestDiffDirRepo(TestDiffDir):
     def test(self):
         result = self.dvc.diff(self.DATA_DIR, a_ref=self.a_ref)
         test_dct = {
-            git.DIFF_B_REF: str(self.a_ref),
-            git.DIFF_A_REF: str(self.git.head.commit),
+            git.DIFF_A_REF: str(self.a_ref),
+            git.DIFF_B_REF: str(self.git.head.commit),
             git.DIFF_LIST: [
                 {
                     git.DIFF_CHANGE: 0,
-                    git.DIFF_DEL: 1,
+                    git.DIFF_DEL: 0,
                     git.DIFF_IDENT: 2,
                     git.DIFF_MOVE: 0,
-                    git.DIFF_NEW: 0,
+                    git.DIFF_NEW: 1,
                     git.DIFF_IS_DIR: True,
                     git.DIFF_TARGET: self.DATA_DIR,
-                    git.DIFF_OLD_FILE: self.DATA_DIR,
-                    git.DIFF_OLD_CHECKSUM: (
-                        "d5782f3072167ad3a53ee80b92b30718.dir"
-                    ),
                     git.DIFF_NEW_FILE: self.DATA_DIR,
                     git.DIFF_NEW_CHECKSUM: (
+                        "d5782f3072167ad3a53ee80b92b30718.dir"
+                    ),
+                    git.DIFF_OLD_FILE: self.DATA_DIR,
+                    git.DIFF_OLD_CHECKSUM: (
                         "bff5a787d16460f32e9f2e62b183b1cc.dir"
                     ),
-                    git.DIFF_SIZE: -30,
+                    git.DIFF_SIZE: 30,
                 }
             ],
         }